Interaction of caffeine and sulfadiazine with lysozyme adsorbed at colloidal metal nanoparticle interface: influence on drug transport ability and antibacterial activity.

The modulated bioactivity of proteins immobilized on nanoparticle (NP) interfaces is of tremendous interest toward designing better therapeutic and diagnostic tools. In this work, binding behavior and the antibacterial activity of free lysozyme (LYS) as well as its non-covalent assembly with silver (Ag) and gold (Au) colloidal NPs were compared in presence of two model drugs, viz. sulfadiazine (SDZ) and caffeine (CAF). Intrinsic protein fluorescence was found to quench due to the formation drug-protein complex in case of CAF resulting a linear Stern-Volmer (SV) plot with K = 1.83 × 10 M.On the other hand, a positive deviation beyond [SDZ] ~0.15 mM is explained due to the formation of a fluorophore - quencher sphere with radius of 13.85 ± 1.80 Å that results almost one order of magnitude higher K (1.75 × 10 M). Molecular docking calculation also predicts relatively more stabilized complex of SDZ with LYS in comparison to CAF (ΔE ~ 3 kJ mol). Synchronous fluorescence results corresponding to Trp and Tyr residues as well as FTIR spectra in the amide I region of LYS confirms minimal deformation in the LYS secondary structure on adsorption to spherical NP surface. Although the nature of LYS-drug interaction remains invariant, the extent of quenching interaction as well as the drug binding ability is strongly modulated in presence of NPs. Further, the antibacterial activity of LYS in presence of the investigated drugs shows 9-14% upsurge with AuNP, in sharp contrast to ca. 31-34% decrease in AgNP.